1. Field of the Invention
The present invention relates to a method of manufacturing a power semiconductor device such as a transistor, a thyristor or a rectifier for great power. More particularly, the invention relates to an improved method of manufacturing the ceramic package of a power semiconductor device. Also, the present invention relates to a semiconductor device manufactured by this improved method.
2. Description of the Related Art
The package of a power semiconductor device is made of ceramics, for two reasons. First, ceramics withstand a very high voltage. Secondly, ceramics are more airtight than a resin, and prevents the semiconductor element sealed within the package from being oxidized and hence from deteriorating.
The ceramic package is an airtight container which comprises a hollow cylinder made of ceramics and a metal seal member closing the open end of the cylinder. Obviously, an airtight sealing must be achieved between the metal seal member, on the one hand, and the open end of the cylinder, on the other.
In order to attain airtight sealing, the end of the ceramic cylinder is metallized, using metals having high melting points. For example, powder comprising Mo, Mn, and W particles as main components is applied to the end face of the ceramic cylinder, and the cylinder is heated to 1400.degree. to 1700.degree. C. in a reducing atmosphere. As a result, the metals react with the ceramic, thus metallizing the end face of the cylinder. Then, a metal seal member is brazed to either metallized end face of the ceramic cylinder.
The method of assembling the ceramic package, descried above, is disadvantageous in the following respects:
First, this method can be applied to oxide ceramic only. Alumina packages have been used widely for encasing semiconductor devices. Recently, packages made of non-oxide ceramic, such as aluminum nitride and silicon carbide, which are greatly thermal conductive, are used in increasing numbers to encase semiconductor devices. There is a demand for a method of assembling ceramic packages, which can be applied not only to oxide ceramic, but also to non-oxide ceramic.
Secondly, the cost of assembling ceramic packages by the method described above is high since the ceramic cylinders must be heated to very high temperatures.
Thirdly, the metals having high melting points, such as Mo, Mn, and W, used in the method are readily oxidized. When a ceramic package made by the method are used in a high-humidity atmosphere, its metallized layers will soon be corroded, jeopardizing the airtight sealing between the ceramic cylinder and either metal seal member. Consequently, the semiconductor elements sealed within the package will be exposed to air and moisture, inevitably having their characteristics deteriorated.